Questions: Action Potential Initiation: Threshold, All-or-None, and Depolarization

Question 1

A neuron at rest (−70 mV) receives two inputs: input A depolarizes it to −52 mV, and input B depolarizes it to −47 mV. If threshold is −50 mV, what happens?

• A. Neither produces an action potential because neither reaches the +40 mV peak of an action potential
• B. Both produce action potentials, but input B produces a larger one because it depolarizes the membrane more
• C. Only input B produces an action potential; input A fails to cross threshold and the membrane returns to rest
• D. Both produce action potentials of identical size, but input B fires with a shorter latency

Answer: (C)

Input A (−52 mV) stays below threshold (−50 mV), so the small Na⁺ channel opening it produces is overwhelmed by resting K⁺ leak, and the membrane returns to −70 mV. Input B (−47 mV) crosses threshold, triggering the positive feedback cycle where Na⁺ channel opening causes depolarization causes more Na⁺ channel opening — and the spike fires to completion. Option A confuses threshold with the peak voltage. Option B violates the all-or-none principle.

Question 2

A researcher applies stimulus A at 1.5× threshold intensity and stimulus B at 4× threshold intensity to the same neuron. What is the expected result?

• A. Stimulus B produces a larger action potential because more Na⁺ channels are recruited by the stronger depolarization
• B. Both stimuli produce identical action potentials, since amplitude is set by the Na⁺ equilibrium potential and channel properties, not by stimulus strength
• C. Stimulus A produces no action potential; only stimulus B exceeds threshold
• D. Stimulus B produces a faster action potential because faster depolarization recruits channels more quickly

Answer: (B)

This is the all-or-none principle in action. Once threshold is crossed — by 1.5× or 4× doesn't matter — the same regenerative cycle fires: Na⁺ channels open, membrane rushes toward the Na⁺ equilibrium potential (~+50 mV), and the spike peaks at the same amplitude. Stimulus strength above threshold affects firing rate (how often spikes fire) or latency (how quickly the first spike fires), but not individual spike size. Neurons encode intensity by varying rate, not amplitude.

Question 3

A stronger stimulus produces a larger action potential because it opens more voltage-gated Na⁺ channels, driving the membrane to a higher peak voltage.

• T. True
• F. False

Answer: False

This is the most common misconception about action potentials. The all-or-none principle states that once threshold is crossed, the action potential is stereotyped — same amplitude, same shape, same duration — regardless of how far above threshold the stimulus was. Amplitude is determined by the Na⁺ equilibrium potential and the kinetics of voltage-gated channels, not by stimulus intensity. A neuron 'tells' its target cells about stimulus intensity through firing rate (spikes per second), not spike size.

Question 4

Neurons can encode the intensity of a stimulus despite the all-or-none nature of individual action potentials by varying the rate at which they fire.

• T. True
• F. False

Answer: True

Since individual spike amplitude is fixed, the only way to signal 'more' is to fire more spikes per second. Stronger stimuli that produce more sustained depolarization cause the neuron to reach threshold repeatedly in quick succession, increasing firing rate. This is frequency coding: sensory neurons signal stronger stimuli with higher firing rates, and motor neurons drive stronger muscle contractions by firing faster. The all-or-none principle is not a limitation — it's the basis of a clean, noise-resistant digital encoding scheme.

Question 5

Why does positive feedback make threshold a critical, non-negotiable switching point rather than a gradual continuum of neural excitability?

Model answer: Positive feedback creates a bistable switch. Below threshold, any Na⁺ channel opening is more than counterbalanced by resting K⁺ leak currents, which pull the voltage back toward rest — a stable equilibrium. Above threshold, the inward Na⁺ current exceeds all outward currents, so each increment of depolarization causes more Na⁺ channel opening, which causes more depolarization — a self-amplifying runaway cascade. The threshold is the unstable tipping point between these two stable states (rest and the fully fired spike). There is no gradual middle ground: below threshold, the system returns to rest; above it, the positive feedback loop drives the membrane fully to its peak.

This bistability is what gives action potentials their reliability. The all-or-none property isn't just a curiosity — it ensures that signals propagating down a long axon don't decay, because each node along the way undergoes the same full regenerative firing. Compare this to a passive electrical cable, where voltage decays continuously with distance. The threshold-positive-feedback system solves the long-distance signaling problem by resetting the signal to full amplitude at each point.